Polymers from Alkenes (OCR AS Chemistry A)

Addition Polymerisation

• Addition polymerisation is one of the most important addition reactions of alkenes which form the basis of the plastic industry

• Addition polymerisation is the reaction in which many monomers containing at least one C=C double bond form long chains of polymers as the only product

  • Just like in other addition reactions of alkenes, the π-bond in each C=C bond breaks and then the monomers link together to form new C-C single bonds

• A polymer is a long-chain molecule that is made up of many repeating units

• The small, reactive molecules that react together to form the polymer are called monomers

• A polymerisation reaction can be represented by a general formula or by using displayed formulae

  • E.g. poly(ethene) and poly(chloroethene) (also known as PVC) are polymers made up of the ethene and chloroethene monomers respectively and are commonly used in making plastics

The general formulae of the addition polymerisation of ethene (1) and chloroethene (2)

The addition polymerisation of ethene (1) and chloroethene (2)

• Just like any other addition reaction of alkenes, addition polymerisation gives only one product

Deducing repeat units

• A repeat unit is the smallest group of atoms that when connected one after the other make up the polymer chain

  • It is represented by square brackets in the displayed and general formula

• In poly(alkenes) (such as poly(ethene)) and substituted poly(alkenes) (such as PVC) made of one type of monomer the repeating unit is the same as the monomer except that the C=C double bond is changed to a C-C single bond

The repeating units of poly(ethene) and poly(chloroethene) are similar to their monomer except that the C=C bond has changed into a C-C bond

Waste polymers & Alternatives

• Polymers provide a readily available, cheap alternative to many metal, glass, paper and cardboard materials in everyday use

• The low reactivity of many polymers makes them ideal for certain uses, e.g. food packaging, but at the same time creates problems with their disposal as a lot of polymers are non-biodegradable

  • There are many published articles about the environmental problems of waste plastic killing marine animals 

• One method of polymer disposal continues to be the use of landfill sites

  • This is not ideal and various initiatives are being introduced aiming to reduce this method of waste disposal in general as well as with specific regard to polymers

Recycling

• Polymer recycling reduces the amount of waste that it going to landfill sites

  • Newer landfill sites can have a recycling point where the new waste is brought before going to into the actual landfill - this is in an effort to reduce the amount of polymers (and other recyclable materials) unnecessarily going into the waste site

• The recycling of polymers can also reduce the use of finite resources

  • Lots of polymers are made from the products of cracking crude oil and it's fractions

• Recycling polymers is a time-consuming process as they have to be sorted into the different categories

  • These categories are usually shown somewhere on the plastic / polymer product with the recycling symbol and numbers or abbreviations for the different polymers, e.g.:

The recycling symbol for the polyethylene terephthalate polymer

• After sorting, the polymers are chopped, washed, dried, melted and then cast into pellets ready for use

  • However, mixed polymers can mean that this process is wasted as its produces an unusable mix of polymers 

• Certain polymers can cause problems when recycling due to their chemical composition, e.g. PVC contains a large amount of toxic chlorine which can be released

  • Modern techniques are overcoming this PVC problem by dissolving the polymer and precipitating out the recycled material 

Combustion

• Some petroleum / natural gas derived polymers are still difficult to recycle

• Since they have a large amount of energy stored within the polymer chains, these polymers can be incinerated

  • This process can then be used to boil water and use the water vapour to turn turbines inside a power station, in a  similar fashion to coal-fired power stations

• This process still causes environmental pollution as the carbon within the polymer can be released as carbon dioxide contributing to global warming

  • Other toxic waste products include hydrogen chloride from the combustion of PVC

Feedstock recycling

• Feedstock recycling is where waste polymers are broken down, by chemical and thermal processes, into monomers, gases and oils

• These products are then used as the raw materials in the production of new polymers and other organic chemicals

• The major benefit of feedstock recycling, compared to other methods of polymer disposal, is that it works with unsorted and unwashed polymers

Bioplastics

• Bioplastics are polymers that are made from plant starch / cellulose, plant oils and plant proteins

  • They provide a renewable and sustainable alternative to the current polymers which are predominantly based on finite resources such as crude oil

Biodegradable polymers

• Biodegradable polymers can be broken down over time by microorganisms

  • Common products from this process include carbon dioxide, water and other organic compounds

• The polyester and polyamide condensation polymers are considered to be biodegradable as they can be broken down using hydrolysis reactions

  • This is a major advantage over the polymers produced using alkene monomers (polyalkenes)

  • When polyesters and polyamides are taken to landfill sites, they can be broken down easily and their products used for other applications

Compostable polymers

• Compostable polymers are commonly plant based

  • Plant starch is being used in the production of biodegradable bin liners

  • Sugar cane fibres are replacing polystyrene in the production of disposable plates and cups

• Compostable polymers degrade naturally leaving no harmful residues

Photodegradable polymers

• Photodegradable polymers contain bonds that are weakened by absorbing light / visible radiation

  • This starts the breakdown of the polymer

• A lot of photodegradable polymers are oil-based 

  • In certain cases, an additive that absorbs light is mixed into the polymer to promote degradation